In a wafer fabrication process, a wafer is commonly subjected to process gases under pressure in the controlled environment of a process chamber. The deposition formation rate on the wafer and the etching removal rate from the wafer depend on the input gas flow rate of the process gases that enter the process chamber encapsulating the wafer.
A vapor deposition system for wafer fabrication generally includes a liquid delivery or injection system for vaporizing a liquid chemical and carrying the vaporized liquid into the deposition process or reaction chamber for wafer processing. A typical liquid delivery system for a chemical vapor deposition process manages the flow of a liquid precursor or reagent, a carrier gas, and one or more other process gases. The liquid precursor is delivered to a vaporization device at a steady flow rate. The carrier gas is delivered to the vaporization device for mixing with the vaporized liquid precursor. The other process gases are combined with the mixture of the vaporized liquid precursor and carrier gas for delivery to the process chamber.
A critical factor in the production of wafers with superior uniformity is the precise control of the flow rate of the delivery of the liquid precursor into the process chamber. Irregularities in the flow rate may cause nonuniformity and erroneous deposition film thickness that adversely affect wafer quality and acceptability. The liquid precursor flow rate is typically generated by a liquid mass flow controller that is governed electronically by a voltage signal, such as that produced in a liquid flow meter. The accuracy of the flow rate produced by the liquid mass flow controller depends on the calibration between the voltage signal and the actual flow rate delivered. Factory calibration of the liquid delivery system is generally approximate with a typical tolerance of up to 10%. Therefore, a flow rate adjustment technique is necessary to compensate for the imprecise factory calibration and to improve wafer uniformity and obtain proper deposition film thickness.
An estimation and verification methodology in U.S. Pat. No. 5,520,969 utilizes the steady state pressure differentials observed in the process chamber to compute a correction constant for controlling the mass flow rate of the liquid precursor. The steady state pressure differentials are obtained from a first pressure rise due to a flow of a carrier gas through the chamber and a second pressure rise resulting from a flow of the carrier gas injected with the vaporized liquid precursor (or vapor precursor) through the chamber.
Although the '969 patent offers a steady state methodology that is relatively simple and accurate compared to prior techniques that are based on measurements of transient parameters, the approach may produce inaccurate results and undesirable effects in the processing system. For instance, the process chamber may be contaminated by the gases (e.g., TEPO) used in the estimation and verification process, and the contamination may adversely affect the deposition process. Pressure sensors and flow control valves connected to the chamber for gas flow measurement and control may have defects or leaks that contribute to processing errors during deposition. In addition, the theoretical basis for the approach set forth in the '969 patent assumes that (1) the mixture of the precursor vapor and carrier gas behaves as an ideal gas; (2) complete vaporization of the precursor liquid; and (3) there is no condensation of the precursor vapor during its flow from the vaporizer to the chamber. The satisfaction of these assumptions ensures that the precursor vapor pressure is proportional to the precursor liquid flow rate. The departure from these assumption in actual experiments contributes to inaccuracies of the estimation and verification methodology.